Reductive decomposition of calcium sulfate



Apnl 30, 1963 T. D. WHEELOCK ETAL I 3,037,790

REDUCTIVE DECOMPOSITION OF CALCIUM SULFATE Filed Sept. 4. 1959 4 Sheets-Sheet 2 ID! W5 D/A-MAXIMUM cI l2 I I0 DESULFURIZATION RATE,%PER. MIN.

INITIAL o 2 o TEMERATURE, F

G l8 l5 .4 I2 I MAXIMUM DESULFURIZATION IO RATE,% PER MIN.

INITIAL I/ o- I o l 2 a 4 5 Is 7 co IN FEED GAS, /NVEN7'0RS" ATTORNEYS.

Apnl 30, 1963 T. D. WHEELOCK ETAL 3,087,790

REDUCT IVE DECOMPOSITION OF CALCIUM SULFATE Filed Sept. 4. 1959 4 Sheets-Sheet 3 8 MAXIMUM DESULFURIZATION 6 RATE, PER MINL 2 3 4 CO IN FEED GAS,

INITIAL 2 DESU LFUR IZAT ION RATE, PER MIN.

so IN FEED GAS, /NVEN7'0R$- By fl dm, 341% f ATTORNEYS.

A ril 30, 1963 T. D. WHEELOCK ETAL 8 REDUCTIVE DECOMPOSITION OF CALCIUM SULFATE Filed Sept. 4. 1959 4 Sheets-Sheet 4 'D= =U- 'U= $0- -D: =U- -Cl= ==D 0= FD- 7 q Ina/WEEK Thoms@ whee/606 /pox ld 1?. o's/60x1- Gain r18 6 Uflitcd States Patent 3,087,790 REDUCTIVE DECOMPOSITION OF CALCIUM SULFATE Thomas D. Wheelock and David R. Boylan, Ames, Iowa,

assignors to Iowa State College Research Foundation,

Ames, Iowa, a corporation of Iowa Filed Sept. 4, 1959, Ser. No. 838,368 Claims.- (Cl. 23-186) This invention relates to the reductive decomposition of calcium sulfate and more particularly to a procedure by which calcium sulfate is reduced to produce lime and sulfur in a readily convertible form, i.e., sulfur dioxide.

It is a general object of this invention to provide a novel procedure for the reductive decomposition of calcium sulfate, such as the naturally occurring gypsum, anhydrite o-r by-product gypsum such as would be produced in the wet process production of phosphoric acid. Another object is to provide a procedure for decomposing calcium sulfate wherein the products of the reaction are provided in a commercially acceptable form. Still another object is to provide a procedure for the decomposition of calcium sulfate which is characterized by minimal formation of calcium sulfide. Yet another object is to provide the optimum conditions for a calcium sulfate decomposition when using reducing gas mixtures. Other objects and advantages of this invention can be seen as this specification proceeds.

The invention will be explained in conjunction with the accompanying drawings in which FIG. 1 is a chart showing the percent desulfurization of calcium sulfate as a function of time and how this occurs at different temperatures;

FIG. 2 is a chart showing calcium sulfide in the byproduct as a function of carbon dioxide in the feed-gas;

FIG. 3 is a chart showing the effect of temperature on 3,087,?90 Patented Apr. 30, 1963 which an alkaline earth sulfate such as calcium sulfate reacts. with a reducing agent such as carbon monoxide at an elevated temperature to produce sulfur dioxide and an alkaline earth oxide such as lime. The decomposition is illustrated by the following reaction:

By controlling the above reaction, particularly relative to the temperature and composition of the gas phase, we have found it possible to produce a high quality lime byproduct which makes the procedure more flexible in application than the present European process. It appears, however, that the possibility of utilizing the reductive decomposition seen in reaction (1) has been disregarded. We attribute this to the fact that the production of the desired end products has been frustrated through the ease with which calcium sulfate is reduced to calcium sulfide by the following reaction:

CaSO +4CO=CaS+4CO (2) Thermodynamic investigation shows that the principal reaction (reaction (l)) is endothermic and is favored by higher temperatures. Formation of calcium sulfide (reaction (2)) is exothermic and is favored by lower temperatures. Above 2100 F. and under equilibrium conditions, calcium sulfide cannot exist in the presence of calcium sulfate because of the following reaction:

For the reactions of interest, calculated values of the heats of reactions and of the logarithms of the equilibrium constants are presented in Table I. To show that a reducing agent is needed for the decomposition of calcium sulfate, equilibrium constants for reaction (4) have also been included, reaction (4) being:

TABLE I.EQUILIBRIUM CONSTANTS AND HEATS OF REACTION Log 10K Heat of Reaction, g. caL/g. mole Reaction 1*, 200 K. 1, 400 K. 1, 600 K. l,200 K. 1, 400 K. 1, 600 K.

(1) CaSO +CO= C11O+SO2+CQ2 0- 31 1.48 2.28 46, 300 43, 400 39, 500 2 oaso,+4oo CaS-l-4C0z 7.92 6. 69 5.66 46, 500 48, 400 41,000 a) 3OaSO OaS=aO+4SOi 6. 6s 0. 77 3. 44 231, 700 222, 200 208, one (4) OaSO CaO+SOz+1l202 7. 42 -4. 51 2. 38 113, 600 110, 600 106,200

the desulfurization rate;

FIG. 4 is a chart showing the desulfurization rate as a function of the carbon monoxide in the feed-gas;

FIG. 5 is a chart showing the maximum desulfurization rate as a function of carbon monoxide in the feedas; g FIG. 6' is a chart showing the initial desulfurization rate as a function of the sulfur dioxide in the feed-gas; and

FIG. 7 is a flow sheet of apparatus which may be employed in the practice ofthe invention.

Although Frasch sulfur is now the leading raw material for the production of sulfuric acid in this country, it is possible that brimstone may be replaced by gypsum and anhydrite. Reserves of these naturally-occurring calcium sulfate minerals are unlimited, but reserves of Frasch sulfur appear to be inadequate.

Even today, sulfuric acid is being produced from anhydrite in several European plants. These plants are based on a process in which anhydrite, coke and shale are heated to a sintering temperature in a large rotary kiln. Sulfur is recovered as sulfur dioxide which is subsequently converted into sulfuric acid, the clinker being used for Portland cement.

The invention here contemplates a simpler process in We have established that the calcium sulfate in the form of gypsum could be quantitatively decomposed by passing a stream of nitrogen over the gypsum heated to temperatures in the vicinity of 2200 F. However, adding :as little as 1% of the gaseous decomposition products, sulfur dioxide and oxygen, to the nitrogen has prevented the decomposition. Thus, the decomposition of calcium sulfate without the use of a reducing agent was economically unfeasible. We found that calcium sulfate would be decomposed in the presence of as much as 7% sulfur dioxide, provided several percent of a reducing gas moiety such as carbon monoxide was also present in the gas mixture (reaction 1). Generally, we have found that the higher the concentration of the reducing gas, the better the production of sulfur dioxide. However, the utility of this information is sharply limited in that increasing amounts of the reducing gas moiety is productive of increased amounts of calcium sulfide. Even where the reaction is carried out in the preferred temperature range of 2l502250 F., appreciable calcium sulfide is developed. Below 2150 F. the desirable -mteraction between the calcium sulfate and calcium sulfide (reaction 3) is not favored, while above 2250 F., the calcium sulfate tends to fuse somewhat and develop a somewhat glassy surface on the particles. This is believed to seal the pores in the solid particles and prevent passage of the gaseous reactants and products of reaction, thereby limiting production. The development of calcium sulfide in the indicated temperature range is attributable to the fact that equilibrium conditions ordinarily do not exist and further that there are a number of undesirable side reactions involving the production of calcium sulfide operative in a system involving the various reactants.

We have found that the undesirable presence of calcium sulfide can be prevented through the fortification of the reducing gas moiety with a moiety effective to suppress the residue sulfide formation. In the example following, this suppressor gas moiety is carbon dioxide and is present in substantial quantities in the gas mixture.

Example I The influence of providing carbon dioxide as part of the reaction gas mixture was investigated by providing a gas containing 3% carbon monoxide, 20% carbon dioxide and 5% sulfur dioxide, the remainder being nitrogen. This was passed through a gypsum bed having a depth of one inch, the particle size being 7 +8 mesh and the mass velocity of the gas being 0.20 lb./sec.-sq. ft. The percent desulfurization at the temperatures of 2110 F., 2200 F., and 2310" F. is seen in FIG. 1 and at the two higher temperatures no appreciable calcium or other alkaline earth sulfide was produced. The gypsum had the composition set forth in Table II.

In this example as well as that following, the reactor was charged with a one inch thick bed of gypsum and suspended inside of a preheated furnace. As the reactor temperature rose, a mixture of sulfur dioxide and air was passed through it to prevent the gypsum from decomposing. As operating temperature was approached, nitrogen and carbon dioxide were added to the mixture. When the reactor temperature had levelled out, the flow of air was stopped and the fiow of carbon monoxide was started. This marked th beginning of a run. At regular intervals of l-lO minutes depending on the rate of decomposition, the reactor weight and other important data were noted. The run was continued until the reactor reached a constant weight. The reactor was slowly withdrawn from the furnace while nitrogen and a small amount of carbon monoxide were passed through it. The solids from the reactor were analyzed for sulfate, sulfide, and calcium. The sulfate was determined gravimetrically according to the method outlined by the American Society for Testing Materials, ASTM Standards Including Tentatives, 1952, part 3; 265-77. Sulfide was determined iodometrically. The sample was treated with an excess of a standard solution of potassium iodate and potassium iodide. Excess of hydrochloric acid was then added and the sample container was closed immediately. After the reaction had subsided, the excess iodine liberated was titrated with standard sodium thiosulfate solution. Calcium was determined by the versenate titration method developed for determining total hardness in water and described by Diehl, Goetz, and I-Iach, Journal of the American Water Works Association, 42, 4048 (1950).

The total desulfurization or conversion of calcium sulfate to calcium oxide was calculated from the composition of the residual solids. By assuming that the instantancous conversion was proportional to the weight lost by the gypsum charge, it was possible to plot the desulfurization curves, such as those appearing in FIG. 1.

The data presented in FIG. 1 also show the effect of temperature on the desulfurization rate. At 2110 F., the gypsum passed through the initial induction period where little or no decomposition occurred. The reaction rate soon increased, and a relatively constant but rapid desulfurization rate was established. At the end, the gypsum was 87% desulfurized and the solids contained 11% calcium sulfide. Increasing the temperature to 2200 F. had the effect of increasing the initial desulfurization rate, but the maximum rate was unaffected. The total desulfurization was increased to almost and no calcium sulfide was found in the solids. When the temperature was raised to 2310 F. the initial rate was the same as for 2200 F. but the subsequent rate of the decomposition was much lower than for 2200 F.

The effect of changing the quantity of carbon dioxide in the reaction gas mixture was studied in the above procedure and results set forth in Example 2 below.

Example II In the same apparatus employed for the results shown in FIG. 1, various combinations of carbon monoxide, carbon dioxide and nitrogen were passed through a gypsum bed again with a mass velocity of 0.20 lb./sec.-sq. ft. The temperature employed Was about the optimum temperature as determined in Example 1. Table III shows the etfects of carbon monoxide and carbon dioxide ratios on the percent calcium sulfide produced.

Remainder nltrogen. b Mean outlet temperature during maximum dos ulfurization rate period.

From the foregoing table, it can be seen that when the gas fed to the reactor contained more than 1% carbon monoxide and no carbon dioxide appreciable amounts of calcium sulfide were produced. The percent calcium sulfide was found to increase with the carbon monoxide concentration. However, adding as little as 5% carbon dioxide reduced the amount of calcium sulfide to insignificance. In general, it can be said that when sulfur dioxide was excluded from the gas to the reactor, the percent calcium sulfide was found to increase with carbon monoxide concentration. When sulfur dioxide was absent, the presence of as little as 5% carbon dioxide prevented the occurrence of calcium sulfide provided the carbon monoxide concentration did not exceed 4% and the temperature was at least 2200 F.

To approximate commercial production conditions, investigations of the effect of the presence of sulfur dioxide were made. This is seen in Example 3 below.

Example III In the reactor described hereinbefore and using the same-sized gypsum and mass velocity, a reaction gas containing 5% sulfur dioxide and varying amounts of carbon dioxide, carbon monoxide and nitrogen was employed. The temperature was generally in the range of 2140-225 0 F. The results of this series of tests are presented in FIG. 2, where can be seen that the calcium sulfide in the solid by-product could be reduced to virtual insignificance (i.e., less than one percent) through the presence of a Percent Percent 002 When a substantial quantity of sulfur dioxide is present, it appears that the carbon dioxide should be present in the quantity about five to six times that of the carbon monoxide.

The slope of curves such as seen in FIG. 1 is the desulfurization rate and this is extremely significant in any commercial operation. A plot of the slopes of the curves such as are seen in FIG. 1 can be found in FIG. 3. The data on which FIG. 3 is based were derived from runs in which the gas composition included 4% carbon monoxide, carbon dioxide, 5% sulfur dioxide and 81% nitrogen. Other than that, the conditions were the same as those followed providing the data employed for developing FIG. 1. In FIG. 3, it can be seen that the initial desulfurization rate was measurable even at 2100' F. and that it increased throughout the temperature range. Between 2100 and 2200 F., the maximum rate almostdoubled, reaching a peak value around 2250 F. The maximum rate fell at higher temperature so that at 2310" F. the rate was less than at 2.100 F. and not much higher than the initial desulfurization rate.

When sulfur dioxide was excluded from the gas fed, the maximum desul-furization rate reached the peak value at about 2150 F. with 4% carbon monoxide and at about 2250" F. with 2% carbon monoxide. The variation in the rate with temperature was much greater with 4% carbon monoxide than for 2%. Again an increase in temperature from 2100 to 2200 F. caused a sharp reduction in the calcium sulfide produced.

The desulfurization rate appears to be a linear function of the carbon monoxide concentration as is illustrated by FIGS. 4 and 5. In FIG. 4 carbon dioxide and sulfur dioxide were included in the gas fed, the gas composition including 20% carbon dioxide and 5% sulfur dioxide, the percentage carbon monoxide being indicated in FIG. 4 with the remainder again being nitrogen. In FIG. 4, the *desulfurization rate curves were derived from the usual S-shaped desulfurization curves, such as are seen in FIG. 1. Both the initial rate and the maximum rate appear to be first order with respect to the carbon monoxide concentration, but the reaction velocity constant is much smaller for the initial rate than for the rate portions of the dcsulfurization curve. This would seem to indicate achange in the reaction mechanism during the batchwise desulfurization of calcium sulfate. It is of interest to note that both the initial and maximum rates converge on zero at 1.8% of carbon monoxide. At the higher carbon monoxide concentration, the maximum rate did not increase proportionally, perhaps because some calcium sulfide was being produced.

In FIG. 5, sulfur dioxide was excluded, the gas fed contained 5% carbon dioxide. This had the effect of increasing the maximum desul-furization rate for small concentrations of carbon monoxide, but the slope of the linear relation-ship between the rate and the concentration was reduced, so that for 4% carbon monoxide, the maximum rates were about the same for both cases. In another case where both carbon dioxide and sulfur dioxide were excluded, the relationship was about the same as that shown in FIG. 5.

Concerning FIG. 6, it appears that for small carbon monoxide concentrations, sulfur dioxide seems to retard the initial rdesulfurization rate. The desulfurization rate wasiniluenced only to a small degree by carbon dioxide concentration, at least for concentrations up to 30%.

For the conditions which produce S-shaped desulfurization curves, the gas mass velocity had opposite eifects on the initial and maximum de-sulfurization rates. When the mass velocity increased from 0.1 to 0.3 lb./sec.-sq. ft, the initial rate decreased and the maximum rate in creased. The increase in the maximum rate can be attributed to an increase in the mass transfer rate in the gasrphase or to an increase in the average carbon monoxide concentration. In other Words, at the higher mass velocities there would have been a smaller change in the carbon monoxide concentration, across the gypsum bed. It is estimated that the increase of the average carbon monoxide concentration could have accounted for over of the increase in maximum rate with mass velocity. Under these conditions transport in the gas phase did not appear to be the dominant rate controlling mechanism.

The desulfurization rate was found increased to a greater extent with mass velocity in the range of 0.1 to 0.3 lb./sec.-sq. it. Where carbon dioxide and sulfur dioxide wereexcluded and 2% carbon monoxide was fed. The increase in the average carbon monoxide concentration with increasing mass velocity would not account for the increase in the desulfurization rate, hence the mass transfer rate could have been rate controlling.

The efiect of particle size affects the maximum desul-furization rate but has little eifect on the initial desulfurization rate. The maximum desulfurization rate was found to decrease rapidly as the particle size increased. In determining the eifect of particle size on the desulfurization rate, a mass velocity of 0.20 lb./sec.- sq. ft. was employed with the temperature in the range 2l60-2220 F., the gas composition being 3% carbon monoxide, 20% carbon dioxide, 5% sulfur dioxide, with the remainder nitrogen. Under these conditions, the initial \desulfurization rates were substantially constant at 17% per minute for particles of a size ranging from .05.20 inch. The rate decreased from about 10% per minute for the maximum desulfurization rate with .05 inch particles to about 4% per minute with 0.20 inch particles.

The particle size can be important in determining the economics of a commercial installation, inasmuch as with smaller particles there is additional cost in grinding. Also, with exceedingly small particles and fluidized bed reactors, lower gas velocities are required so that a larger contactor is needed.

From the foregoing discussion it can be seen that the composition of the gas phase in the reaction zone is important to the successful operation of the process. For best results in the reaction zone the concentration of carbon monoxide should lie in the range of about 17% and the concentration of carbon dioxide should be about five or more times the concentration of carbon monoxide. Since carbon monoxide is being consumed and carbon dioxide is being generated by the reaction, it follows that the gas fed may contain a larger concentration of carbon monoxide and a smaller concentration of carbon dioxide than is optimum for the reaction zone. This is especially true of the type of reactor where the gas phase is Well mixed so that the incoming gas is mix-ed and diluted with the gas in the reactor. If a concentration gradient reactor (i.e., counter current flow or parallel flow reactors) is employed, the carbon monoxide concentration in the gas fed may not greatly exceed the concentration recommended for the reaction zone. However, the carbon dioxide concentration may be somewhat less providing no substantial amount of sulfur dioxide is present. In the gas entering a con centration gradient reactor, the ratio of carbon dioxide to carbon monoxide need not be as high as in the gas leaving because the sulfur dioxide concentration is very small at the gas inlet compared to the outlet.

The desired ratio of the gas moiety effective to suppress residual sulfide development to the reducing gas moiety can be developed through the incomplete combustion or" reducing gas or coke. If natural gas is employed in the production of the reducing gases, some Water vapor is necessarily present as a combustion product and it is believed that some benefit may be derived from the combination of water vapor and carbon dioxide. A typical natural gas burned with 75-95% of the air as required for complete combustion yields satisfactory amounts of the suppressor gas moiety and the reducing gas for the operation of the invention.

In the use of natural gas burned with insufficient air for complete combustion as a source of the reactant gas, it is important to note that hydrogen if present appears to be more likely to yield calcium sulfide than does carbon monoxide. Further, the side reactions productive of calcium sulfide increase with the concentration of the reducing gas. However, by maintaining in the reaction zone the reducing gas in the range of 17% of the gas mixture and providing a quantity of the suppressor gas moiety of about five or more times the quantity of reducing gas, the tendency toward residual sulfide production is minimized. The best results are obtained when the temperature of the reaction is within the range of 21502250 F. In this temperature range, in the gas fed to the reactor, the mole ratio of reducing gas moiety (i.e., based on carbon monoxide or hydrogen) to calcium sulfate should lie in the range of 1:1 to 2:1 and the mole ratio of sulfide suppressor moiety (carbondioxide) to the calcium sulfate should lie in the range of 1:1 to 6:1.

As mentioned hereinbefore, it is possible to reductively decompose gypsum, which is a by-product of Wet process phosphoric acid. In such a case, the phosphogypsum could be available in the form of a Water slurry. A typical phosphogypsum might contain about 6% silica and 80% of the gypsum might pass through a 100 mesh Tyler sieve.

For the case where the invention is to be applied to the decomposition of phosphogypsum, the apparatus arrangement of FIG. 7 may be employed. There it is to be seen that the gypsum slurry entering at 110 is directed to a spray dryer 111. The dehydrated solids are conveyed to a pair of fluidized bed reactors 112 and 113. As shown, each reactor 112 and 113 contains eight stages. The upper two stages comprise a solids preheating section, while the lower stages comprise a reaction section. In the latter section, the calcium sulfate reacts with a reducing gas moiety such as carbon monoxide and hydrogen produced by burning natural gas in burners 112a and 113a with a limited amount of air. The gas is provided through line 114, While the air for burners 112a and 11311 is provided by pipe lines 115 and 116, respectively, these lines being equipped with blowers 115a and 1160. The lime produced, together with the silica initially present in the phosphogypsum, is conveyed to storage as at 117. The hot sulfurous gas is cooled and purified.

For the purpose of separating solids from gases, the reactors for the initial two stages are equipped with cyclones 118. The gas issuing from the cyclones 118 is :sent to heat exchangers 119 and thence to a stack as at 11911. The solids portion of the material introduced into the cyclones 118 is recirculated to the reactors 112 and 113.

The gaseous portion from the six reaction stages of each reactor 112 and 113 is directed to the spray dryer 111 and the gaseous portion of this to a cyclone 120. The gas from cyclone .120 is directed to heat exchangers 121. The output of these heat exchangers is sent to a dust precipitator 122, a scrubber 123, and a mist precipitator 124, after which the gas is sent to a sulfuric acid plant through line 125.

The arrangement illustrated above offers several advantages. It permits feeding gas having a difierent composition to each stage of the reactor 112 or 113, as the case maybe. Thus, in the upper two stages, an oxidizing gas can be used for preheating solids so that no calcium sulfide is formed. In the lower stages, the reducing gas moiety and the sulfide-suppressor moiety can be adjusted for each stage to give best overall results. Further, a multiplicity of stages insures more uniformity in the time individual particles of solids remain in the reactor. Still further, sulfur dioxide is removed from each stage as soon as it is liberated. In this fashion, there is a minimal tendency for side reactions to occur which might reduce sulfur dioxide to sulfur or hydrogen sulfide, which in turn might react with the lime to form calcium sulfide.

The arrangement presented also provides advantageous operation in the scrubber and condenser arrangement. After the warm gas passes through the dust precipitator 122, it still contains some dust and considerable water vapor, which must be removed. By employing the arrangement shown, in which the scrubber 123 has its gas outlet 123a connected to a condenser or heat exchanger 126 and a separator 127, it is possible to condense the water vapor and use the resulting liquid to scrub out the dust. The water vapor is condensed by heat exchange with cooling water provided by line 128. Thereafter, the condensed water vapor is returned to the scrubber 123 as through line 127a. Some sulfur dioxide will be absorbed in the cool condensate, but this loss is minimized in that the scrubber also acts as a stripper for the sulfur dioxide. The liquid is heated as it passes through the scrubber 123, which reduces the solubility of the sulphur dioxide so that it can be desorbed. Thus, We have provided an advantageous method for purifying a warm gas stream of suspended dust and condensable vapor without significant loss of any other components which may be soluble in the condensed vapor. For the practice of the invention in the case of natural gypsum or anhydrite, the apparatus also may be arranged as in FIG. 7. The differences in raw material properties may make minor modifications desirable. For example, it may prove more economical to feed natural gypsum or anhydrite in the form of dry solids which are of a larger particle size than the usual phosphogypsum. Under these circumstances, a spray dryer could be eliminated and fewer reactor stages might be required.

While in the foregoing specification We have set forth a detailed description of the invention for the purpose of explanation thereof, it will be understood that those skilled in the art may make many variations in the details herein given without departing from the spirit and scope of the invention.

We claim:

1. In the gaseous reduction of calcium sulfate to provide calcium oxide substantially free of calcium sulfide, the step of contacting at a temperature in the range of 2l50-2250 F. said calcium sulfate with a gaseous mixture comprising a reducing gas, CO and an inert gas, said contacting being carried out in the presenue of 50;, said reducing gas constituting about 17% of said mixture, with said CO being present in an amount greater than the combined amounts of said reducing gas and S0 2. In the gaseous reduction of calcium sulfate to provide calcium oxide substantially free of calcium sulfide, the step of contacting in the presence of sulfur dioxide and at a temperature in the range of 2l502250 F. said calcium sulfate with a gaseous mixture comprising:

a reducing gas, carbon dioxide, water vapor, and an inert gas,

said reducing gas constituting at least about 1% of the sulfur dioxide-containing atmosphere in the reaction space surrounding said calcium sulfate,

with the sum of said carbon dioxide and water vapor being present in said atmosphere in an amount at least five times the amount of said reducing gas.

3. The process of claim 2 in which said reducing gas constitutes from about 1% to about 7% of said atmosphere.

4. The process of claim 2 in which the said reducing gas constitutes from about 3% to about 5% of said atmosphere.

5. In the gaseous reduction of calcium sulfate to provide calcium oxide substantially free of calcium sulfide, the step of contacting in the presence of sulfur dioxide and at a temperature in the range of 21502250 F. said calcium sulfate with a gaseous mixture comprising:

a reducing gas, carbon dioxide, and an inert gas,

said reducing gas constituting at least about 1% of the sulfur dioxide-containing atmosphere in the reaction space surrounding said calcium sulfate,

with said carbon dioxide being present in said atmosphere in an amount at least five times the amount of said reducing gas.

6. The process of claim 5 in which said reducing gas is carbon monoxide and constitutes at least about 2% of said atmosphere.

7. The process of claim 6 in which said carbon monoxide constitutes from about 3% to about 5% of said atmosphere.

8. In the gaseous reduction of calcium sulfate to provide calcium oxide substantially free of calcium sulfide, the step of contacting in the presence of sulfur dioxide and at a temperature in the range of 2150-2250 F. said calcium sulfate with a gaseous mixture comprising:

carbon monoxide, hydrogen, carbon dioxide, water vapor, and an inert gas,

the sum of said carbon monoxide and hydrogen constituting from about 1% to about 7% of the sulfur dioxide-containing atmosphere in the reaction space surrounding said calcium sulfate,

with the sum of said carbon dioxide and water vapor in said atmosphere being present in an amount at least five times the sum of said carbon monoxide and hydrogen.

9. In the gaseous reduction of calcium sulfate to provide calcium oxide substantially free of calcium sulfide, the step of contacting in the presence of sulfur dioxide and at a temperature in the range of 2150-2250 F. said calcium sulfate with a gaseous mixture comprising:

a reducing gas, carbon dioxide, water vapor, and an inert gas, said reducing gas constituting from about 1% to about 7% of the sulfur dioxide-containing atmosphere in the reaction space surrounding said calcium sulfate,

with the sum of said carbon dioxide and water vapor in said atmosphere being at least five times the sum of said reducing gas and sulfur dioxide.

10. In a method of treating calcium sulfate to provide sulfur dioxide and calcium oxide, the steps of flowing a carbon dioxide-containing reducing gas in a plurality of parallel streams while flowing finely-divided calcium sulfate in a fluidized stream generally perpendicular to the direction of flow of said reducing streams, contacting the reducing gas and calcium sulfate streams at a temperature in the range of 21502250 F., the carbon dioxide being present in a quantity greater than the reducing gas, and separating the calcium oxide from the sulfur dioxide to provide calcium oxide substantially flee of calcium sulfide.

References Cited in the file of this patent UNITED STATES PATENTS 1,845,744 Fleck Feb. 16, 1932 2,449,537 Hixson Sept. 21, 1948 2,589,684 Edge Mar. 18, 1952 2,740,691 Burwell Apr. 3, 1956 FOREIGN PATENTS 764,537 Great Britain Dec. 28, 1956 OTHER REFERENCES .Altieri, book Gas Analysis and Testing of Gaseous Materials, pages 32, 33 and 36 (1945 ed.), Mack Printing Co., Easton, Pa. 

1. IN THE GASEOUS REDUCTION OF CALCIUM SULFATE TO PROVIDE CALCIUM OXIDE SUBSTANTIALLY FREE OF CALCIUM SULFIDE, THE STEP OF CONTACTING AT A TEMPERATURE IN THE RANGE OF 2150-2250*F. SAID CALCIUM SULFATE WITH A GASEOUS MIXTURE COMPRISING A REDUCING GAS, CO2, AND AN INERT GAS, SAID CONTACTING BEING CARRIED OUT IN THE PRESENCE OF SO2, SAID REDUCING GAS CONSTITUTING ABOUT 1-7% OF SAID MIX- 